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bpf: Fix out-of-bounds read of rtt_min in sock_ops

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A sockops prog reading skops->rtt_min never checks the sk type: on the
tcp_conn_request() path sock_ops->sk is a request_sock (non-full), and the
ctx rewrite casts it to a tcp_sock (full) and reads rtt_min past the end of
the request_sock, returning dirty adjacent memory.

	SEC("sockops")
	int prog(struct bpf_sock_ops *skops)
	{
		switch (skops->op) {
		case BPF_SOCK_OPS_RWND_INIT:
			leak = skops->rtt_min;   /* reads the request_sock OOB */
		...
		}
	}

For instance one such read returned rtt_min=0xffff8881, the high half of a
leaked kernel pointer.

Guarding that cast is exactly what SOCK_OPS_GET_FIELD() does -- it checks
is_locked_tcp_sock and returns 0 when sock_ops->sk is not a locked full
socket. Every other tcp_sock field in sock_ops goes through it; rtt_min is
the only one open-coded, so it skips the check.

Read rtt_min through SOCK_OPS_GET_FIELD() too. rtt_min is a bit special:
it is a struct minmax and we only want the current min, so pass
rtt_min.s[0].v. That is equivalent to the old hand-computed offset

	offsetof(struct tcp_sock, rtt_min) + sizeof_field(struct minmax_sample, t)

(s[0] sits at rtt_min + 0 and .v at + sizeof(.t), i.e. what minmax_get()
returns), so the loaded field is unchanged and only the full-sock guard is
added. The two BUILD_BUG_ON()s that protected the hand-computed offset
are no longer needed.

Before patch:

	0: r1 = *(u64 *)(r1 +0)      ; r1 = skops->sk
	1: r1 = *(u32 *)(r1 +2324)   ; ((tcp_sock *)sk)->rtt_min.s[0].v

After patch:

	0: *(u64 *)(r1 +56) = r9
	1: r9 = *(u8 *)(r1 +50)      ; is_locked_tcp_sock
	2: if r9 == 0 goto pc+4      ; not a locked full sock -> 0
	3: r9 = *(u64 *)(r1 +56)
	4: r1 = *(u64 *)(r1 +0)      ; r1 = skops->sk
	5: r1 = *(u32 *)(r1 +2324)   ; rtt_min.s[0].v
	6: goto pc+2
	7: r9 = *(u64 *)(r1 +56)
	8: r1 = 0

Fixes: 44f0e43037 ("bpf: Add support for reading sk_state and more")
Reported-by: VEGA <vega@nebusec.ai>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260903100921.113374-1-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Jiayuan Chen 2026-09-03 18:09:20 +08:00 committed by Alexei Starovoitov
parent 85136bf224
commit 75f8cf2246
1 changed files with 1 additions and 12 deletions
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13
net/core/filter.c
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@ -11106,18 +11106,7 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
break;
case offsetof(struct bpf_sock_ops, rtt_min):
BUILD_BUG_ON(sizeof_field(struct tcp_sock, rtt_min) !=
sizeof(struct minmax));
BUILD_BUG_ON(sizeof(struct minmax) <
sizeof(struct minmax_sample));
*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
struct bpf_sock_ops_kern, sk),
si->dst_reg, si->src_reg,
offsetof(struct bpf_sock_ops_kern, sk));
*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
offsetof(struct tcp_sock, rtt_min) +
sizeof_field(struct minmax_sample, t));
SOCK_OPS_GET_FIELD(rtt_min, rtt_min.s[0].v, struct tcp_sock);
break;
case offsetof(struct bpf_sock_ops, bpf_sock_ops_cb_flags):